Lubricants for use with air compressors, engine oils, gear oils, hydraulic fluids, and the like, require excellent characteristics of high viscosity index, good lubricity, high oxidation stability, and high thermal stability. Oxidation stability is important due to repeated and prolonged exposure of the lubricant to air, various metallurgies, and sealing materials. A desirable lubricant composition remains in a liquid phase over a wide temperature range, has good low temperature fluidity, has a low vapor pressure, and is operable over an extended period of time at wide ranging temperatures and pressures. Viscosity at high temperatures should be sufficient to provide adequate lubrication, and at low temperatures should be low enough to allow start-up of the compressor at subzero temperatures without the need for external heating.
In addition to its effect on the useful service life of the lubricant itself, the oxidation stability of the lubricant also affects the performance of the compressor equipment. One of the most difficult lubricant-related problems encountered in compressor equipment is the formation of carbon deposits within the compressor and associated piping. This is caused by oxidation of lubricant contained in the air stream as it passes through the equipment.
Compressor lubricants, as well as lubricants used in other applications, are often brought into direct and intimate contact with gas. This contact generally occurs at elevated temperatures and pressures, and is repetitive. Where the gas coming in contact with the lubricant is air, the oxygen content of the air in combination with the high pressure and high temperature presents an oxidizing atmosphere that is very severe. Hence, lubricants with enhanced oxidation stability are greatly needed.
Enhanced oxidation stability is also highly desired in other applications, such as in the lubrication of gear boxes, bearing sets, hydraulic systems and chain drives. When oxidation is problematic in these types of applications, sludge and carbon/varnish deposits appear on the metal surfaces, which may adversely impact the functioning of the equipment, and result in increased downtime and higher maintenance costs. Additionally, lubricants and fluids used in gear boxes, bearing sets, hydraulic systems and chain drive applications are expected to survive for long drain intervals; therefore, increased service life of the lubricant is desired.
Lubricant composition adapted for high temperature applications have been described previously. For example, U.S. Pat. No. 4,175,045 to Timony describes a synthetic lubricant composition comprising a polyol ester of a carboxylic acid having from about 4 to about 13 carbon atoms in its structure. Further, U.S. Pat. No. 6,436,881 to McHenry et al. describes a synthetic polyol ester based lubricant that has a base stock that is the reaction of a polyol mixture including a major proportion of dipentaerythritol, and a mixture of monocarboxylic acids. Conventional lubricants that are composed of some level of synthetic or natural esters can show evidence of limited oxidation stability while in service in air compressor and other applications, thus leading to limited service life. Accordingly, there is a need for lubricant compositions that provide improved oxidation stability when exposed to moisture and high temperatures. The present invention addresses these needs, among others.